Complete mitochondrial genome of the hydrozoan jellyfish Blackfordia virginica Mayer, 1910 (Cnidaria; Hydrozoa; Leptothecata) with phylogenetic analysis

Abstract In this study, we analyzed the complete mitochondrial genome of the hydrozoan jellyfish Blackfordia virginica. The genome was a linear form (15,109 bp long, 73.6% AT), including 13 protein-coding genes (cox2, atp8, atp6, cox3, nad2, nad5, nad6, nad3, nad4L, nad1, nad4, cytB, and cox1), 2 tRNAs (tRNA-Met and tRNA-Trp), and 2 rRNAs (12S and 16S RNA). The genome structure of the B. virginica was completely identical to mitochondrial genomes of other hydrozoans that belonged to Leptothecata and Anthoathecata. Molecular phylogenetic analysis within hydrozoan species showed that B. virginica was the closest to the hydrozoan Laomedea flexuosa.


Introduction
Leptothecata (thecate hydroids) is an order of hydrozoans in the phylum Cnidaria. They show great morphological variations among species according to their mode of development, growth stages, and defensive structures (Maronna et al. 2016). The hydrozoans have a complex life cycle, including a polyp stage, a medusa stage, or both, and their polyps are always living in colonies which grow rapidly on rocks and shells. Based on the morphology, their taxonomy has been described for a long time (Cornelius 1995a(Cornelius , 1995b, and until now approximately 2300 species of Leptothecata have been documented in public database (WoRMS 2020). Recent molecular phylogenetic approaches have improved the deep relationships among species of the Leptothecata, suggesting the addition of new clades in this order (Maronna et al. 2016). Considering the huge numbers of species, there are insufficient molecular data for a more accurate classification of the true relationship.
The hydrozoan jellyfish Blackfordia virginica Mayer, 1910 (Cnidaria; Hydrozoa) is a member of the Leptothecata (WoRMS 2020). It was first described from the Black Sea and has been considered a native species there. However, to date, it is considered an invasive species due to the worldwide expansion via trading by ships (Mills and Sommer 1995). In this study, we first described and analyzed the complete mitochondrial genome structure of B. virginica. In addition, molecular phylogenetic analysis was performed using five hydrozoans, including three Leptothecata.
The specimen of B. virginica was collected from Songsangyo (37 12 0 22.4 00 N, 127 01 0 24.2 00 E) in South Korea, on 7 July 2020. Genomic DNA was extracted from the whole body by using the cetyl trimethylammonium bromide (CTAB) method (Richards et al. 2003) and the remaining part of the specimen was stored in the Department of Biotechnology, Sangmyung University, Korea (Accession No. EN424). The whole mitochondrial genome was sequenced on MGISEQ-200 platforms, and paired-end reads of mitogenome sequences were assembled and annotated using Getorganelle version 1.7.1a (Jin et al. 2020), MITOS (Bernt et al. 2013), and Geneious version 9.1.3 (Geneious, Auckland, New Zealand), respectively. A molecular phylogeny tree was constructed based on concatenated amino acid sequences of 13 proteincoding genes (PCGs) in MEGA X (Kumar et al., 2018). The molecular phylogenetic analysis method has been described in our previous study (Karagozlu et al. 2019).
The phylogenetic relationships of the class Hydrozoa were investigated (Figure 1). The molecular phylogenetic tree showed that B. virginica was clustered with other Leptothecata species, and L. flexuosa is the closest species to B. virginica. The genome sequence data in this study provide additional data for phylogenetic classification among hydrozoan species.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This research was a part of the project titled 'Improvement of management strategies on marine disturbing and harmful organisms [No. 20190518]

Data availability statement
The data that support the findings of this study are openly available in GenBank with the accession number MW376866 (https://www.ncbi.nlm. nih.gov/nuccore/MW376866). Figure 1. Molecular phylogenetic tree of Hydroidolina. The tree was constructed with the concatenated amino acid sequences of 13 mitochondrial protein coding genes using the maximum-likelihood algorithm (JTT matrix-based model) with 1000 bootstrap replicates. A black dot represents Blackfordia virginica determined in this study.